Preheat oven to 180˚C. Heat 1 tablespoon oil in a frying pan over a medium heat. Add onion and pancetta. Cook, stirring, for 5 minutes, or until golden. Place in a bowl to cool. Combine herbs, garlic, mustard and remaining oil in another bowl. Reserve 2 tablespoons of herb mix to rub on beef. Add sourdough and remaining herb mix to onion. Season with salt and pepper.

Slice a 10cm pocket in beef, between bones and flesh. Stuff with sourdough mix. Truss with kitchen string. Heat an oiled roasting pan over a high heat. Season beef well. Sear for 10 minutes, turning regularly, until browned all over. Rub with remaining herb mix. Roast beef for 1 hour 15 minutes for medium, or until cooked to your liking. Meanwhile, wrap beetroot in foil. Roast for 50 minutes, until tender. Remove foil, peel and cut in half. Set aside.